Tell them that they are the strongest people in the world. Make sure to lie to them if there will be any needles or any pain involved. Tell them it won't even hurt. And if they do you should congratulate them! make sure too:
1. prepare them before the visit.
2. don't create any fears or joke about needles.
3. Don't make any empty promises
4. Offer distractions
5. Be comforting
6. Offer a small treat afterward